1. Field of the Invention
The present invention relates to a method of manufacturing a non-shrinking multilayer ceramic substrate, and more particularly, to a method of manufacturing a non-shrinking multilayer ceramic substrate, which can prevent a conductive via connecting interlayer circuits from protruding and inhibit the formation of a void in a via hole in manufacturing a multilayer ceramic substrate through a non-shrinkage process.
2. Description of the Related Art
In general, a multilayer ceramic substrate employing glass-ceramic has high flexibility in terms of design because it allows the implementation of interlayer circuits having a three-dimensional structure. Multilayer ceramic substrates are increasingly utilized in the market of smaller and higher-performing high frequency components. With multilayer ceramic substrates becoming more complicated and precise, inner patterns and via structures have less margin in design and thus non-shrinkage sintering is required to suppress transverse shrinkage of the multilayer ceramic substrate.
To this end, a soluble green sheet of a sinter-resistant material, which is not sintered at a sintering temperature of a ceramic substrate material, is bonded to at least one of two surfaces of a non-sintered ceramic laminate in order to suppress the shrinkage of the ceramic laminate in the x-y direction.
However, for the electrical connection between interlayer circuits, a plurality of via holes are formed in a multilayer ceramic substrate obtained by laminating a plurality of ceramic green sheets on top of each other. The via holes are filled with a conductive electrode material.
A conductive via, formed of conductive metal powder, an organic binder and a solvent, shrinks in volume due to the sintering process. Because the conductive metal powder shrinks to a greater extent than the ceramic in the sintering process, the via hole and the conductive via are separated from each other due to the different shrinkage rates thereof, creating a large void in the via hole, even if the via hole is completely filled with the conductive electrode material before the sintering process.
Particularly, the conductive via, when sintered, shrinks in the circumferential direction and thus shrinks less in the thickness direction because of the green sheets which serve to inhibit shrinkage in the non-shrinkage process but have small shrinkage inhibiting effects on the non-sintered ceramic laminate. As a result, the conductive via becomes higher than the via hole after the sintering process, thereby protruding to the outside and creating a void in the via hole.
To prevent the formation of the void in the via hole resulting from the sintering process, the via hole may be filled with an excessive amount of conductive electrode material exceeding the volume of the via hole in the green state. However, this causes the conductive electrode material to flow over the via hole during the laminating or pressurizing process, and thus results in short circuits between layers of the substrate or the peeling of the layers, degrading the product yield.
FIGS. 1A and 1B are cross-sectional views illustrating examples of defects in a conductive via after non-shrinkage sintering in the manufacturing of a non-shrinking multilayer ceramic substrate according to the related art. A non-sintered multilayer ceramic substrate may be obtained by forming via holes in ceramic green sheets through punching or the like, filling the via holes with conductive electrode paste, and then laminating and thermally compressing the ceramic green sheets. Shrinkage inhibiting green sheets made of sinter-resistant powder are bonded to two surfaces of the non-sintered multilayer ceramic substrate. Thereafter, non-shrinkage sintering is performed on the resultant structure, thereby fabricating a multilayer ceramic substrate.
In non-shrinkage sintering, the constraining force of the shrinkage inhibiting green sheet is weaker on and around the conductive via than on the ceramic green sheet. This is because the material contacting the shrinkage inhibiting green sheet in the conductive via, which breaks the continuity of the ceramic green sheet, is the conductive electrode paste of the conductive via, not a low temperature co-fired ceramic material. Consequently, defects occur around the conductive via after sintering because of the difference in shrinkage behavior in the circumferential direction.
Accordingly, as shown in FIG. 1A, avoid (A) is generated in the via hole as the conductive via shrinks in the circumferential direction because of the small constraining force acting thereon from the shrinkage inhibiting green sheet. Also, the shrinkage of the conductive via is less than desired in the thickness direction due to its shrinkage in the circumferential direction. For this reason, the conductive via becomes relatively thicker than the multilayer ceramic substrate, thereby protruding above the substrate after sintering.
As for another example of the defect, as shown in FIG. 1B, sinter-resistant powder, serving to inhibit the shrinkage of the conductive via, is added to obstruct the volume shrinkage of the conductive via, so that the formation of a void can be prevented from occurring. However, since the sinter-resistant powder is used to inhibit the shrinkage of the conductive via, the low density (C) of the conductive via occurs due to an organic binder and a solvent added to the conductive electrode paste.